The deposition of organic contaminants (i.e., pitch and stickies) on surfaces in the papermaking process is well known to be detrimental to both product quality and the efficiency of the papermaking process. Some contaminating components occur naturally in wood and are released during various pulping and papermaking processes. Two specific manifestations of this problem are referred to as pitch (primarily natural resins) and stickies (adhesives or coatings from recycled paper). Pitch and stickies have the potential to cause problems with deposition, quality, and efficiency in the process as mentioned above.
The term “pitch” can be used to refer to deposits composed of organic constituents which may originate from these natural resins, their salts, as well as coating binders, sizing agents, and defoaming chemicals which may be found in the pulp. In addition, pitch frequently contains inorganic components such as calcium carbonate, talc, clays, titanium and related materials.
“Stickies” is a term that has been increasingly used to describe deposits that occur in the systems using recycled fiber. These deposits often contain the same materials found in “pitch” deposits in addition to adhesives, hot melts, waxes, and inks.
The deposition of organic contaminants, such as pitch and stickies, can be detrimental to the efficiency of a pulp or paper mill causing both reduced quality and reduced operating efficiency. Organic contaminants can deposit on process equipment in papermaking systems resulting in operational difficulties in the systems. The deposition of organic contaminants on consistency regulators and other instrument probes can render these components useless. Deposits on screens can reduce throughput and upset operation of the system. Deposition can occur not only on metal surfaces in the system, but also on plastic and synthetic surfaces such as machine wires, felts, foils, Uhle boxes and head box components.
From a physical standpoint, “pitch” deposits have usually formed from microscopic particles of adhesive material (natural or man-made) in the stock which accumulate on papermaking or pulping equipment. These deposits can readily be found on stock chest walls, paper machine foils, Uhle boxes, paper machine wires, wet press felts, dryer felts, dryer cans, and calendar stacks. The difficulties related to these deposits included direct interference with the efficiency of the contaminated surface, therefore, reduced production, as well as holes, dirt, and other sheet defects that reduce the quality and usefulness of the paper for operations that follow like coating, converting or printing.
From a physical standpoint, “stickies” have usually been particles in the stock which originate from the recycled fiber. These deposits tend to accumulate on many of the same surfaces that “pitch” can be found on and causes many of the same difficulties that “pitch” can cause. The most severe “stickies” related deposits tend to be found on paper machine wires, wet felts, dryer felts and dryer cans.
Methods of preventing the build-up of deposits on the pulp and paper mill equipment and surfaces are of great importance to the industry. The paper machines could be shut down for cleaning, but ceasing operation for cleaning is undesirable because of the consequential loss of productivity, poor quality of the paper while the machine is partially contaminated, and “dirt” which occurs when deposits break off and become incorporated in the sheet. Preventing deposition is thus greatly preferred where it can be effectively practiced.
Historically, the subsets of the organic deposit problems, “pitch” and “stickies”, have manifested themselves separately, differently and have been treated distinctly and separately. This was true because mills usually used only virgin fiber or only recycled fiber. Often very different treatment chemicals and strategies were used to control these separate problems. However, current trends are for increased mandatory use of recycled fiber in all systems. This results in a co-occurrence of stickies and pitch problems in a given mill.
The use of clay and clay-like materials for controlling the deposition of organic contaminants in the pulp and paper making processes is known to those skilled in the art. For example, the use of talc in such processes is described by Kirk and Othmer, Encyclopedia of Chemical Technology, (New York, N.Y., John Wiley & Sons, Inc., 1983), volume 22, page 529.
The clay, or clay-like material, is usually delivered in powder form to pulp and paper mills, where it is dispersed into water to make a suspension, which is then typically fed to the pulp slurry in the papermaking systems. The use of a clay, or clay-like material, in pulp and paper mills has some drawbacks: 1) it requires capital investment for the feed and mixing equipment that is required to handle the clay, or clay-like material, powder and to disperse it into water, 2) powder can cause dust in the air which can be a health hazard, and 3) handling and feeding a powder in a make-down operation can be labor-intensive. Therefore, for the application of controlling pitch and stickies deposition in papermaking systems, it is highly desirable to have the clay, or clay-like material, in a slurry form to be delivered to pulp and paper mills.
Several factors must be considered when developing a clay, or clay-like material, slurry product for the pulp and paper industry. The slurry product must have reasonably high solids content in order to be economical. However, the viscosity of such a slurry increases exponentially with its solid content and high viscosity is not desirable for handling. Also, clay, or clay-like material, slurry without any stabilizer is unstable against sedimentation and will settle rather quickly. For example, the precipitate from clay slurry can become dense and compact and form a “hard cake” which can be rather difficult to re-disperse. Furthermore, if the slurry is intended for use for organic contaminant control in papermaking systems, it requires that all additives such as dispersant and/or suspending agents added to the clay, or clay-like material, slurry must have no detrimental effect on the function of the slurry in reducing and preventing organic deposition.
Unfortunately, most traditional dispersants or suspending agents used to disperse clay, or clay-like material, slurries such as polycarboxylates, polysulfonates, polyphosphates, sodium tripolyphosphate, sodium hexametaphosphate, sodium silicates, and the like, carry negative charges, which may reduce the affinity of clay to pitch and stickies contaminants and render the material less effective as a pitch and stickies control agent. Therefore, there is a need to prepare a clay, or clay-like material, slurry which has a high solids content, low viscosity, good stability against sedimentation, and efficacy for organic contaminant control.
An aqueous slurry of a mixture of calcined and hydrated kaolin clay (china clay) comprising a suspending agent and dispersant is taught in U.S. Pat. No. 4,017,324. It was further taught in '324 that suitable suspending agents include hydroxyethyl cellulose and methyl cellulose, suitable dispersants include non-ionic surface active agents, and the materials exhibited minimal settling after 2 to 4 weeks. U.S. Pat. No. 4,187,192 teaches that talc, unlike china clay, is relatively hydrophobic and attempts to prepare aqueous talc dispersions have not been entirely successful. It is further taught in '192 that the limitations of the prior art regarding the preparation of an aqueous talc slurry can be addressed by the use of an anionic wetting agent. U.S. Pat. No. 6,074,473 teaches that the viscosity of a talc solution in a “makedown” process can be depressed by formulation with a salt. Talc slurry resistant to foaming comprising a carboxymethyl cellulose wetting agent and an anionic polyacrylate dispersing agent are taught in U.S. Pat. No. 6,267,811 B1 to be stable for a period of a few days. The stability of the slurry of '811 can be improved by the further addition of a xanthan gum stabilizing agent. Talc slurries that are highly stable during transport and storage comprising at least one multivalent ion and a chelant are taught in U.S. Pat. No. 7,258,732 B2.